Single-mol. fluorescence spectroscopy of a multichromophoric conjugated polymer (mol. wt. ∼ 20,000) revealed surprising single-step photobleaching kinetics and acute jumps in fluorescence intensity. These jumps were shown not to result from spectral diffusion and were attributed to fluctuations in the quantum yield of emission for the mols. The data indicate efficient intramol. electronic energy transfer along the polymer chain to a localized fluorescence-quenching polymer defect. The defects are created by reversible photochem. of the polymer. These findings have implications for the use of conjugated polymers in light-emitting diode displays and sensors. [on SciFinder(R)]